1. Field of the Invention
The present invention relates to a storage battery and, more particularly, to an electrode used for an alkaline storage battery and a method for producing the same.
2. Description of the Prior Art
A typical positive electrode of an alkaline storage battery is a nickel electrode. This electrode may be a sintered type electrode or a non-sintered type electrode. In producing the former type of electrode, a microporous sintered plaque obtained by sintering nickel powder is impregnated with an aqueous solution of nickel nitrate or the like thereby to add nickel salt, and, after drying, the sintered plaque is immersed in caustic alkali aqueous solution to convert the nickel salt to nickel hydroxide. This method has the disadvantage that the process is complicated and the filling density of nickel hydroxide as an active material is reduced in comparison with the non-sintered electrode described later. In spite of this disadvantage, this electrode has a highly efficient discharge characteristic and a long cycle life, and finds wide application in a variety of fields.
A non-sintered electrode has previously been of a pocket type. According to a method recently put into practice, on the other hand, nickel hydroxide powder as a powder of active material directly fills a foamed-nickel porous material. This method is a simple method of electrode production. Further, the availability of a foamed nickel porous material of high porosity makes it possible to fill it with nickel hydroxide to a high density and therefore a high-capacity battery can be produced. The foamed nickel porous material, however, needs to be produced by electroplating and therefore has the disadvantage of high material cost.
In view of this, a non-sintered electrode is under development using a low-cost punched metal or expanded metal in place of the foamed nickel porous material as an electrode support. These electrode supports have no three-dimensional structure unlike the sintered plaque or the foamed-nickel porous material. As a result, an electrode made of these electrode supports has a low ability to hold an active material and the active material is liable to fall off during electrode fabrication or repeated charging and discharging. Further, due to the low electronic conductivity in the electrode thickness direction and a poor electrode characteristic, these electrode supports find no practical applications except for special types of electrodes.
The above-mentioned method of electrode production using a punched metal or expanded metal as an electrode support has the advantage that a powder of the active material made into a paste with a solution of a high polymer binder and a conductive powder is coated and dried on the electrode support and thus the electrode can be easily produced. The adhesion between the metal substrate acting as the electrode support and the active material layer is generally weak so that the active material is liable to peel off from the metal substrate in an application using the electrode for batteries. In the case where the electrode support acts as a current collector, the electrical resistance of the electrode increases thereby causing a reduced discharge voltage and discharge capacity. In order to solve this problem, adding a great amount of binder to the active material layer suppresses the separation. The resultant reduced reactivity of the active material, however, has an adverse effect on the discharge characteristic.
In a method for strengthening the adhesion between the metal substrate and the active material layer, a thermoplastic resin layer functioning as a binder is formed on the surface of the metal substrate. Then, the active material is coated on the thermoplastic resin layer and the electrode is heated, to improve the adhesion between the metal substrate and the active material layer. This method, however, has a disadvantage that an insulating layer is formed between the metal substrate and the active material layer with the result that the current collecting characteristic of the electrode is reduced, thereby reducing the reactivity of the electrode.
As described above, these problems are difficult to solve when a comparatively flat metal substrate is used as an electrode support.